To force Vegas to use Sony's own, I guess. As the help file says: "When this check box is selected, the Vegas application will ignore DV codecs other than its own proprietary codec" - Wouldn't this ensure that Vegas was ONLY using codecs that had been tried and tested by Sony? However, having the option to allow them access to Vegas would imply that you COULD try them out too, but you would have too do this on purpose by unchecking this box.